JP3226199B2 - Hex bolt made of austenitic stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hexagon bolt (hereinafter, simply referred to as "fastening bolts"), and more particularly to a fastening bolt used in a corrosive environment.

[0002]

2. Description of the Related Art Stainless steel fastening bolts are manufactured by forging unlike fastening bolts made of ductile cast iron. Although there is a cast steel product by a casting method even in stainless steel, the productivity is considerably lower than that of forging, and if the C% is relatively high to increase the castability, a problem may occur in terms of corrosion resistance. In this regard, if a stainless steel bar is forged in a mold, fastening bolts having the same shape can be efficiently mass-produced. Therefore, continuous forming by a forging press is usually mainly used.

In this case, the material of stainless steel is as follows:
JIS standard SUS30 which is a typical stainless steel
4 (austenitic) or SUS403 (martensitic), and the bar is heated to 1000 ° C. or higher in a heating furnace and hot forged to finish it into a desired shape.

[0004]

As described above, in the typical forging of stainless steel, hot forging is commonly used in order to enhance the formability as much as possible. If molding is performed cold, the material is not sufficiently moldable, and if it exceeds the limit of the allowable molding rate, it cannot withstand the material even if it slightly exceeds the limit. We face the challenge of not being met. As a factor that further aggravates this problem, conventional stainless steel fastening bolts, such as SUS304, have a large deformation resistance in the cold state. Therefore, a large deformation stress is required for plastic deformation. There is a high possibility that so-called work-induced transformation occurs along with the deformation, and the austenitic phase transforms into a martensite phase, which significantly deteriorates the originally possessed corrosion resistance, which is likely to spur the inherent properties of austenitic stainless steel.

[0005] When the fastening bolts are formed by a forging press hot, the material is at least about 1000.
Since it is heated to ℃ and forged, not only heating equipment is required, but also the fuel cost and work cost naturally increase. Of course, it is needless to say that the working environment and occupational safety are unfavorable.

[0006] Despite the many advantages of cold or at least warm forming of the fastening bolts, one of the key elements that has hindered its practice is the cold forming interior. Due to the residual stress, it induces work-induced transformation and extremely deteriorates corrosion resistance, and stress corrosion cracking and pitting, which are peculiar corrosion phenomena of austenitic stainless steel, occur during long-term use of products. Finally, the life of the product is finally cut off. Stress corrosion cracking is likely to occur when hot water containing halogen is encountered in a state where internal stress remains, and may occur even with slight residual stress and slight chlorine ion. In addition, pitting is a phenomenon in which the passivation in a specific small range is destroyed and corrosion progresses, and is often caused by the presence of halogen ions such as chlorine. It is said that pitting corrosion can cause unexpectedly large damages due to many factors such as stagnation of liquid, corrosion products, and accumulation of foreign substances.

[0007] In order to remove internal stress, solution treatment from around 1000 ° C, which is performed on many stainless steels, is the most commonly used means. Due to this heat treatment, the stress at the time of working deformation preserved inside disappears, and the directionality of the crystal flowing at the time of molding changes in the austenitic system due to the processing history in the austenitic system due to recrystallization. It goes without saying that the effect of remarkably improving and extending the service life can be obtained.

However, it cannot be denied that this solution treatment is a factor that considerably lowers the material strength obtained by cold working. The strength of the solution-treated austenitic stainless steel is markedly lower than that of martensitic stainless steel or general ferritic steel, and the characteristic that the proof strength is particularly low as compared with tensile strength is conspicuous. Even if the strength reaches an appropriate level as a structural material after being formed in a cold or at least a warm state, there is a concern that the possibility of corrosion peculiar to stainless steel due to the internal stress as described above is concerned, and this concern is eliminated. Therefore, when a heat treatment for removing stress is applied, a decrease in strength, which is a cause of structural instability, and particularly a remarkable decrease in yield point is caused. While lowering the yield point has an advantage during molding, there is no doubt that the final product will incorporate noticeable weaknesses during use.

In order to solve the above-mentioned problems, the present invention uses austenitic stainless steel capable of being cold-formed and has high resistance to corrosion peculiar to the material, and furthermore, a part of the structural material is used. It is an object of the present invention to provide a fastening bolt which bears a reliable level in terms of strength and a manufacturing method thereof.

[0010]

The fastening bolts made of austenitic stainless steel according to the present invention are as follows: C: 0.06% or less, Si: 0.60% or less, Mn: 2.00% or less, P: 0.045%.
%, S: 0.03% or less, Ni: 8.00 to 12.00%, C
r: 17.00 to 20.00%, Cu: 1.32 to 1.40 %, N: 0.07 to
The austenitic stainless steel material consisting of 0.115 % and the remaining Fe is forged in a cold or at least 300 ° C. or less hot region, and subjected to a solution treatment after forming to provide both high corrosion resistance and improved proof stress. Solved the problem.

[0011]

[0012]

FIG. 1 is a table plotting the relationship between the working ratio and the deformation stress when the material is worked cold during the operation of the present invention. In the figure, the ordinate represents the deformation stress (Kgf / mm ² ), the abscissa represents the compressibility (%), and the solid line is SUS3 which is a comparative example.
The curve indicated by 04 is shown by a hatched area in which the embodiments of the present invention are collectively displayed. From this chart, it can be seen that the material of the present invention has a much lower deformation resistance than the stainless steel that has been conventionally and typically adopted, and that a higher deformation rate can be obtained with the same stress. . The improvement of the moldability in the cold depends largely on the action of Cu, but N has the action of increasing the strength and hardness, so that the balanced coexistence of both components is an important requirement of the present invention. As long as it is included in the component limitation of the present invention, as can be seen from the drawing, it has become a basis for easily performing cold forging of fastening bolts, which was conventionally difficult.

The material characteristic of the present invention lies in the balance between Cu% and N%. The addition of Cu% is extremely effective in improving the formability of the material, but the effect cannot be achieved as intended unless it exceeds 1.00%. But,
Its effect is more than 4.00% reached saturation, Although the material a is between the primary molding a bar heat forging for fastening bolts becomes difficult, 1.32 ~ range confirmed in Example
1.40 % is optimal.

On the other hand, the incorporation of N% in a certain ratio is not only effective for stabilizing the austenite phase, as is the case with Cu, but also, as is well known, in addition to the refinement of crystal grains, It has been confirmed that it has a significant effect on improving the low yield point, which is a problem of stainless steel.
N forms an interstitial solid solution in the parent phase and strongly forms an austenite phase. Materials for melting stainless steel (scrap,
Ferroalloys and refractory of melting furnaces, etc.
In the present invention, 0.07 to 0.115 % is intentionally added to improve the basic strength of the parent phase, and the heat treatment for removing internal stress is performed. It complements the accompanying decrease in proof stress. Compounded N
Forms nitrides and carbonitrides with components such as C and Cr to form clusters in the matrix and fix dislocations, and also suppresses the aggregation of precipitated carbides that contribute to corrosion by slowing the diffusion rate of C. . That is, unlike C, N maintains the corrosion resistance despite improving the yield point, and thus can exert the effect to achieve the object of the present invention. Although the addition of N has been reported in prior art in which a considerable amount of Ni is added as an alternative component to Ni in austenitic stainless steel, the subject of the present invention is fastening bolts.
0.07% is necessary for the effect of the addition of N to be remarkable, but if it exceeds 0.115 %, the formability at the time of cold is reduced, and the forging of the fastening bolts at the time of cold or at a low temperature can be performed with proper Cu. Even when blending is involved, reliable work without defects is no longer difficult and within the range confirmed in the examples is 0.115
% Was optimally set as the upper limit.

[0015]

FIG. 2 is a perspective view of an embodiment of the present invention.
The nominal diameter of the hexagonal bolt 1 is 20 mm (JIS M20 type), the parallel spacing of the head 11 is 30 mm, the length of the shaft 12 is 85 mm, and the range of the external thread 13 on the outer peripheral surface of the shaft is 46 mm. As a first embodiment of the present invention for forming this hexagon bolt,
The chemical components are C: 0.05%, Si: 0.56%, Mn: 1.59%,
P: 0.030%, S: 0.002%, Ni: 8.70%, Cr: 18.5
Austenitic stainless steel comprising 0%, Cu: 1.40%, N: 0.078%, and the remaining Fe was formed by cold forging and further subjected to a solution treatment at 1000 ° C. Further, in Example 2, the chemical components were C: 0.06%, Si: 0.
44%, Mn: 1.67%, P: 0.007%, S: 0.004%, N
i: 8.80%, Cr: 18.70%, Cu: 1.32%, N: 0.115
%, And a hexagonal bolt having the same shape as that of Example 1 was formed at a temperature of 200 ° C. using an austenitic stainless steel consisting of the remaining Fe, and a solution treatment was performed at 1000 ° C.

These comparative examples are fastening bolts having the same shape and the same dimensions as those of the hexagonal bolts of the above embodiment, and have chemical components of C: 0.06%, Si 0.50%, Mn: 1.01%, P: 0.032.
%, S: 0.005%, Ni: 8.19%, Cr: 18.38%, C
A material consisting of an austenitic stainless steel consisting of u: 1.50%, N: 0.03% and the balance of Fe and having a balance of Cu: N outside the limited range of the present invention was cold forged and subjected to a solution treatment.

In order to obtain a conclusion on mechanical properties, tensile test pieces were cut out from each of Example 1, Example 2, and Comparative Example, and the mechanical properties were confirmed. Table 1 shows the results.

[0018]

[Table 1]

As shown in Table 1, it can be confirmed that the difference in material strength between the example and the comparative example is remarkable. In particular, the improvement of proof stress, which is a weak point of the austenitic stainless steel as a structural material, has been proved to be a remarkable improvement. The main component difference between the two materials is the N content, which is a very clear basis for applying the material as fastening bolts.

If the fastening bolts of the present invention are applied, a great improvement in proof stress can be expected, and therefore, in the case of the first embodiment, the bolts are replaced with standard products one size smaller than those of the conventional austenitic stainless steel fastening bolts. It is also possible. That is, the guaranteed load of the bolt is determined to be expressed by the product of the cross-sectional area at the root diameter of the thread portion and the yield strength. For example, a standard hexagon bolt M20 (root diameter 17.2) is used.
94 mm), about 48 KN calculated from the resistance of the comparative example.
6 (valley diameter 13.835 mm) and the guaranteed load is calculated from its resistance to be about 48 KN. It is recognized as a common general theory that even if the size is reduced to fastening bolts one rank lower than in the prior art, they can be sufficiently replaced.

[0021] On the other hand, compatibility with corrosion resistance is an extremely important factor for the purpose of using stainless steel, in contrast to the advantage of using it as a part of a structural material. For the confirmation, a corrosion test was carried out under the same conditions (test days: 150 days) for the above-mentioned Examples of the present invention and Comparative Examples, and Table 2 summarizes the results.

[0022]

[Table 2]

As shown in this table, in various corrosive environments, the examples of the present invention are not inferior in corrosion resistance to the comparative examples, but show rather good results. That is,
Significant improvement of material strength mainly by improvement of resistance is preserved despite the solution treatment, and internal stress due to solution treatment induces high corrosion resistance, and resistance and corrosion resistance are improved. Demonstrating that both coexist.

[0024]

As described above, according to the present invention, the fastening bolts according to the present invention have a stable base which is unlikely to induce the corrosion phenomena inherent to austenitic stainless steel, and have a strength, in particular, a part of the structural material. The low level of resistance, which is a concern when incorporated into a device, has also been greatly improved. In addition, since the fastening bolts that are the subject of the present invention have excellent formability such that they can be forged in a cold or at least a warm state, a number of steps and equipment when hot forging is forced are omitted. However, not only quality but also economic competitiveness can be positioned at an immovable level.

[Brief description of the drawings]

FIG. 1 shows the results of an example of the present invention by a chart of deformation stress and compressibility.

FIG. 2 shows a front view (A) and a side view (B) of a hexagon bolt standard product which is an object of the present invention.

 ────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-56-169756 (JP, A) JP-A-50-81908 (JP, A) JP-B-51-20288 (JP, B2)

Claims (1)

(57) [Claims] 1. C: 0.06% or less, Si: 0.60% or less, M
n: 2.00% or less, P: 0.045% or less, S: 0.03% or less,
Ni: 8.00 to 12.00%, Cr: 17.00 to 20.00%, Cu:
An austenitic stainless steel material consisting of 1.32 to 1.40 %, N: 0.07 to 0.115 %, and the remaining Fe is forged in a cold or at least 300 ° C or less hot region, and is subjected to a solution treatment after forming to have high corrosion resistance. Hexagonal bolt made of austenitic stainless steel characterized by having both proof strength.